 /*******************************************************************************
  * Copyright (c) 2007 IBM Corporation and others.
  * All rights reserved. This program and the accompanying materials
  * are made available under the terms of the Eclipse Public License v1.0
  * which accompanies this distribution, and is available at
  * http://www.eclipse.org/legal/epl-v10.html
  *
  * Contributors:
  * IBM Corporation - initial API and implementation
  ******************************************************************************/

 package org.eclipse.ui.internal;

 import org.eclipse.core.expressions.Expression;
 import org.eclipse.jface.util.IPropertyChangeListener;
 import org.eclipse.jface.util.PropertyChangeEvent;
 import org.eclipse.ui.PlatformUI;
 import org.eclipse.ui.internal.services.IEvaluationReference;
 import org.eclipse.ui.internal.services.IEvaluationService;

 /**
  * This internal class serves as a foundation for any handler that would like
  * its enabled state controlled by core expressions and the IEvaluationService.
  *
  * @since 3.3
  */
 public abstract class AbstractEvaluationHandler extends AbstractEnabledHandler {
     private final static String PROP_ENABLED = "enabled"; //$NON-NLS-1$
 private IEvaluationService evaluationService;
     private IPropertyChangeListener enablementListener;
     private IEvaluationReference enablementRef;

     protected IEvaluationService getEvaluationService() {
         if (evaluationService == null) {
             evaluationService = (IEvaluationService) PlatformUI.getWorkbench()
                     .getService(IEvaluationService.class);
         }
         return evaluationService;
     }

     protected void registerEnablement() {
         enablementRef = getEvaluationService().addEvaluationListener(
                 getEnabledWhenExpression(), getEnablementListener(),
                 PROP_ENABLED, null);
     }

     protected abstract Expression getEnabledWhenExpression();

     /**
      * @return
      */
     private IPropertyChangeListener getEnablementListener() {
         if (enablementListener == null) {
             enablementListener = new IPropertyChangeListener() {
                 public void propertyChange(PropertyChangeEvent event) {
                     if (event.getProperty() == PROP_ENABLED) {
                         if (event.getNewValue() instanceof Boolean ) {
                             setEnabled(((Boolean ) event.getNewValue())
                                     .booleanValue());
                         } else {
                             setEnabled(false);
                         }
                     }
                 }
             };
         }
         return enablementListener;
     }

     /*
      * (non-Javadoc)
      *
      * @see org.eclipse.core.commands.AbstractHandler#dispose()
      */
     public void dispose() {
         if (enablementRef != null) {
             evaluationService.removeEvaluationListener(enablementRef);
             enablementRef = null;
             enablementListener = null;
             evaluationService = null;
         }
         super.dispose();
     }
 }

